The present invention relates to a noise reducer and noise reduction method useful for reducing impulse noise and other noise in, for example, a car radio.
Impulse noise such as ignition noise and electric-mirror noise occurs frequently in the car environment, and is readily picked up by the antenna of a car radio. (Electric-mirror noise occurs when an electrically driven mirror is raised or lowered.) The audible effects of such noise in the audio output of a car radio are a familiar experience to automobile riders. To prevent impulse noise from becoming audible, a car radio requires an impulse noise reducer.
A conventional impulse noise reducer operates by detecting each noise impulse, and blanking out the noise by holding the output audio signal constant, or performing some type of interpolation. Further details of the conventional noise-reduction process will be given later.
A problem is that impulse noise is detected in relation to the general noise floor. When a strong broadcast signal is received, the noise floor is low, and substantially all of each noise impulse can be detected. When a weak broadcast signal is received, however, the noise floor rises, and a considerable portion of each noise impulse may fail to be detected. The undetected parts are not blanked out, so the impulse noise, although reduced, remains audible in the output signal.
This problem is not limited to impulse noise, but occurs when any type of noise is blanked out by the method described above.
An object of the invention is to provide improved noise reduction in a demodulated signal, under both weak and strong received electric-field conditions.
A further object is to avoid unnecessary distortion of the demodulated signal.
The invented noise reduction method removes noise from a demodulated signal by the following steps:
(a) determining the electric-field strength of a radio signal from which the demodulated signal has been obtained;
(b) comparing the electric-field strength with at least one predetermined reference value, thereby generating an indicator signal indicating the electric-field strength;
(c) detecting noise in the demodulated signal;
(d) generating gate pulses responsive to the indicator signal and to detection of the noise; and
(e) modifying the demodulated signal during intervals indicated by the gate pulses, thereby reducing the noise.
Step (c) preferably includes the output of pulses indicating noise detection. Step (d) then includes changing lengths of these noise-detection pulses by different amounts according to the indicator signal, preferably changing the lengths by increasing amounts as the electric-field strength decreases.
Changing the pulse lengths in this way can improve noise reduction performance by removing more of the noise, especially when the received electric field is weak.
Step (d) may be carried out by expanding the noise-detection pulses by different amounts to generate a plurality of expanded signals, delaying the expanded signals by different amounts, and selecting one of the delayed expanded signals as a gate signal. The delays enable the gate pulses to be correctly aligned with the impulse noise.
Step (e) may be carried out by storing a certain number of samples of the demodulated signal in a memory, and altering the stored sample values. A variety of alterations can then be carried out in a simple way.
The invented method may also include steps of detecting the overall amplitude level and the high-frequency amplitude level of the modified demodulated signal, and comparing these two amplitude levels. When the high-frequency amplitude level exceeds the overall amplitude level, step (d) is omitted; the demodulated signal is modified during the intervals indicated by the noise-detection pulses. Distortion of high-frequency components of the demodulated signal is thereby avoided.
The invented method may also include steps of detecting the noise level in the demodulated signal, and comparing the noise level with a lower limit. When the noise level is less than the lower limit, step (d) is omitted; the demodulated signal is modified during the intervals indicated by the noise-detection pulses. Unnecessary distortion of the demodulated signal due to over-correction is thereby avoided.
The invented method may also increase the amounts by which the pulse lengths are changed in step (d) when the noise level exceeds a predetermined value, thereby compensating for the spreading of strong impulse noise due to other processing of the demodulated signal.
The invention also provides impulse noise reducers employing the methods described above.